The strength of a transmitted signal is directly determined by the performance of an antenna. In adjustment of the antenna, impedance matching and Voltage Standing Wave Ratio (VSWR) have a great influence on the performance of the antenna. Particularly, Total Radiated Power (TRP), Total Isotropic Sensitivity (TIS) and antenna efficiency are extremely important indexes of a mobile phone, which affect the performance of the entire mobile phone and determine the success or failure of a product.
The impedance matching is a working state in which load impedance and internal impedance of an excitation source are interactively adapted to obtain the maximum power output. The impedance matching reflects power transfer relation between an input circuit and an output circuit. When the circuit impedance matches, the maximum power transfer can be obtained. When the impedance mismatches, the power is reflected, the transmitted energy is reduced, and the circuit performance is also damaged.
With diversity of mobile phone systems and popularization of multi-band Code Division Multiple Access (CDMA)/Wideband Code Division Multiple Access (WCDMA)/Global System for Mobile Communications (GSM) mobile phones, the conventional mobile terminals do not only work at a certain independent frequency band. In daily use, people increasingly find that the performance varies more widely along with wider coverage of the frequency band. Since the antenna impedance matching has different values for different frequency bands, when the antenna is in a mismatched state, the call performance is poor, the power consumption of the mobile phone is high, the standby time is short and the body burns after continuous use. The mobile phone design requirement of the GSM has brought greater challengers to antenna matching design of smart phones.
Meanwhile, the structure of the mobile phone also diversifies from bar phones in the beginning to slide phones, side slide phones and 360-degree rotatable displays at present. Such a condition results in that the antenna radiation direction of a mobile phone changes along with change in angle of use. The radiation reference surfaces of the same mobile phone in a back-forth sliding close state and an open state are also different. However, antenna adjustment is generally guaranteed in a certain single state preferably, therefore, it is enormously difficult to choose the antenna matching form and variable.
Furthermore, in design of antenna matching of the mobile phone, generally the radiation performance of a free space is tested, but the influence on the human head and hand is rarely considered, as a result, the harm caused to human bodies cannot be reduced to the minimum under different service environments and conditions. How to adjust the antenna matching of the mobile phone at the right moment according to application conditions and reduce a Specific Absorption Rate (SAR) value on the premise of guaranteeing the receiving performance also becomes an urgent problem to be solved at present.
Meanwhile, with respect to a structural process, although the material and structure of the front and rear shells of the mobile phone, and spray painting and thermal plating processes may change, the form of the antenna is fixed. Therefore, the antenna matching is also required to be adaptively adjusted. With respect to an assembly process, the press fit degree of an antenna elastic sheet differs and the grounding degrees of a mainboard and the antenna differ, therefore, different requirements on the impedance matching are also caused. Finally, with respect to a production process, certain accuracy error also exists in the current antenna Flexible Printed Circuit (FPC) process, frequently fine difference exists in FPC thickness, corners, slits of the same batch of antennas, the slight difference causes difference in antenna radiation performance of one batch of mobile phones, and all these differences can be calibrated and compensated through the antenna impedance matching.
At present, the antenna matching of the mobile phone has the following modes: 1, a single passive L type or π type matching network is adopted; 2, a plurality of matching units are combined and one of the matching units is selected for matching through a path selector or a switch control; 3, antennas with two or more than two working modes are available and each antenna is independently matched; and 4, a variable capacitor is controlled through voltage control or Automatic Gain Control (AGC) to realize matching.
However, all the above-mentioned methods have some disadvantages. For example, in the mode 1, the matching form is fixed and multi-band adjustment is unfavourable. In the form 2, only different matching forms can be selected, the matching forms are complex and redundant and are also relatively independent and fixed for each combination form, and no fine adjustment function is available. In the mode 3, a fixed matching device is provided for each antenna and the mode 3 is only used for a multi-antenna radio frequency system. In the mode 4, the impedance matching is adjusted on the basis of the conventional variable capacitor, the adjustment range is narrow, the adjustment capacity is weak and the matching requirements of a plurality of application occasions at present cannot be met.